The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Aspects of invention relate to a disconnectable mooring assembly for a vessel, comprising a mooring buoy and a swivel positioned above the mooring buoy, wherein the mooring buoy is provided with a central member for being anchored to the seabed and comprising a number of passages each adapted for receiving a riser, the mooring buoy further comprising an outer member surrounding the central member and capable of a rotation relative thereto, which outer member is adapted to be housed in and locked to a corresponding receiving opening of the vessel.
Such a disconnectable mooring assembly allows a vessel to weathervane around the mooring buoy for minimising loads resulting from external factors, such as wind, current and waves. Under severe conditions (for example at the arrival of a hurricane) the mooring buoy can be disconnected from the vessel and the vessel can navigate to a safe location.
The swivel of such a mooring assembly provides a rotating connection between the risers of the mooring buoy and corresponding lines on board of the vessel, which have a variable position relative to each other as a result of the vessel weathervaning around the mooring buoy (more specifically the central member of the mooring buoy) which basically is kept geostatic by anchor lines connecting the mooring buoy to the seabed.
In a known disconnectable mooring assembly of the above type the swivel is connected directly on top of the mooring buoy (more specifically the central member thereof) and supported thereby in the operational position of the mooring assembly.
Such a state of the art configuration, however, has a number of drawbacks. For connecting and disconnecting (e.g. a controlled lowering through wire/hoisting means) the outer member of the mooring buoy to and from, respectively, the vessel the respective region of the vessel (which generally is located near the keel of the vessel) has to be accessible, such as for (de)bolting, cleaning and inspection. This means that the swivel has to be moved to a position away from the mooring buoy. This calls for complicated moving mechanisms and when the swivel is moved upward or sideward such a movement requires a respective receiving space to be defined in the vessel as well as flexible lines that need to be disconnected or that need to be able to cope with an upward and/or sideward moving swivel (typically with less reliable jumper hoses); in addition the moving mechanisms require a high position flexibility and position accuracy for re-connection of the (typically heavy) swivel on top of the mooring buoy.
Similar disadvantages are valid for the connections between the mooring member and the swivel. In the operational position of the mooring assembly (swivel connected to and supported by the mooring buoy) the presence of the swivel makes the establishment or disconnection of such connections difficult, whereas an inspection and maintenance of such connections is very troublesome. In addition, space between the mooring buoy and the swivel is known to be small and is troublesome for inspection, maintenance and manifold features, such as control racks and pig handling (the need for pig launchers and/or receivers between mooring buoy and swivel).
A further disadvantage is, that during operation the weight of the swivel is supported by the mooring buoy, which adds to the loads on the bearing between the central member and the outer member of the mooring buoy, and indirectly to the loads acting on the vessel at said region.